Algal polyunsaturated fatty acids (PUFAs) are essential to aquatic ecosystems as high-energy, often-essential resources. Some PUFAs and their derivatives are known to function in marine chemical ecology. More recently it is become apparent that these compounds play similar roles in freshwaters. High PUFA cell content in some algal taxa has been linked with their production of derivatives that act as of pheromones, allelogens or toxins. To date, these chemical interactions have been documented for a small number of diatom and chrysophyte species. This paper will demonstrate that these phenomena likely are far more widespread. It will present a series of field and laboratory studies of planktonic and periphytic communities from a diversity of systems and show that these communities are rich sources of certain PUFAs and derivatives, which may function in both positive and negative foodweb interactions. This also has important implications for water quality, as these compounds are potent sources of rancid fishy odours.